Scanning electron microscope

ABSTRACT

The present invention relates to a scanning electron microscope employing a deceleration field forming technology (retarding), more particularly a scanning electron microscope which separates and detects secondary electrons at high efficiency.  
     The object of the present invention is accomplished by providing an electron source, a lens for condensing the primary electron beam which is emitted from said electron source, a detector for detecting electrons which are generated by radiation of the primary electron beam onto a specimen, a first deceleration means for decelerating the primary electron beam which is radiated onto said specimen, a second deceleration means for decelerating electrons which are generated on the specimen, and a deflector for deflecting said electrons which are decelerated by said second decelerating means.

FIELD OF THE INVENTION

[0001] The present invention relates to a scanning electron microscope,more particularly to a scanning electron microscope for obtaininghigh-resolution scanning images at a low acceleration voltage.

BACKGROUND OF THE INVENTION

[0002] A scanning electron microscope (hereinafter shorted as SEM inthis specification) obtains a magnified 2 dimensional scanning image ofan object to be examined by emitting a beam of electrons from a heatingtype or a field emission type, concentrating it by an electrostatic ormagnetic lens into a fine electron beam (a primary electron beam),applying the electron beam onto the object to be examined in a2-dimensional scanning manner, detecting a secondary signal electronsthat are secondarily generated or reflected from the object, and feedingthe magnitude of the detected signal to the brightness modulation of aCRT tube which is scanned in synchronism with the primary electron beam.

[0003] A conventional SEM emits electrons from an electron source towhich a negative voltage is applied, accelerates them by anodes of agrounding voltage, and applies the accelerated electrons (a primaryelectron beam) to an object to be examined at a grounding voltage.

[0004] Recently, semiconductor chips have become smaller and theircircuit patterns have become extremely fine. Accordingly, SEMs have beenwidely used in place of optical microscopes to inspect manufacturingprocesses of semiconductor chips and processed semiconductor chips (e.g.by measurement of dimensions by electron beams and inspection ofelectric operations),

[0005] Semiconductor specimens to be examined by the SEM are generallymade of a multiple layers of electrically-insulating materials on aconductor such as aluminum or silicon. When an electron beam is appliedto such a specimen, the surface of the specimen is charged up, whichwill change the direction of motion of the emitted secondary electronsor the primary electrons themselves. Consequently, the resulting imagesmay have an extraordinary contrast, distortion, or the like. To reducethe influence by this charging up, the energy of the emitted electronbeam must be made as low as possible.

[0006] However, if the energy of the emitted electron beam (anacceleration voltage) is made low, a chromatic aberration due to energydispersion of the electron beam generates and the resolving power of theSEM drastically goes down, which makes observation at a highmagnification harder.

[0007] As a means for solving such a problem, a technology for forming adeceleration field for electron beams (hereinafter called “retarding” inthis specification) has been disclosed in Japanese Non-examined PatentPublication No.06-139985 (1994).

[0008] “Retarding” is a technology of forming a deceleration field byincreasing the voltage to accelerate the electron beam to the anodes andapplying a negative potential to the object to be examined, finallysetting the acceleration voltage to a comparatively low level, and thuspreventing chromatic aberration and charging up.

DISCLOSURE OF INVENTION

[0009] Although this retarding technology can reduce the charging-up ofa specimen and accomplish low chromatic aberration and high resolvingpower, it has the following demerits:

[0010] While the deceleration field which is formed by applying anegative voltage to the specimen can decelerate the motion of theprimary electron beam, it accelerates the secondary electrons and thereflected electrons generated by the specimen. In other words, thesecondary electrons as well as the reflected electrons are refracted tothe electron beam with a high energy.

[0011] Japanese Non-examined Patent Publication No.06-139985 (1994) hasdisclosed a technology as a means for detecting electrons having such ahigh energy. This technology places a micro channel plate (MCP) havingan aperture to pass the primary electron beam with its detection surfaceopposite to the specimen. This technology also places an energy filterwhich selectively detects reflected electrons between the specimen andthe detecting surface of the micro channel plate.

[0012] Some other technologies have also been disclosed. One of suchtechnologies has been disclosed in Japanese Non-examined PatentPublication No.09-171791 (1997). This technology causes high-energyelectrons to collide against a reflecting plate, converts them intosecondary electrons, then guides the secondary electrons into adetector. Japanese Non-examined Patent Publication No.08-124513 (1996)has disclosed another technology-which detects electrons by strikingelectrons directly against an electron multiplier tube.

[0013] However, as the scanning electron microscopes using thesedetection principles detect both secondary electrons and reflectedelectrons by an identical detector. Accordingly, this type of SEM cannotdistinguish secondary electrons from reflected electrons clearly.

[0014] Although the technology disclosed in Japanese Non-examined PatentPublication No.06-139985 (1994) can detect reflected electrons only orboth secondary and reflected electrons by means of the energy filter, itcan neither detect only secondary electrons which are accelerated by adeceleration field nor detect both secondary and reflected electronsindependently and simultaneously without mixture of information specificto secondary and reflected electrons.

[0015] The secondary electrons and the reflected electrons respectivelyhave specific information. Therefore, it has been desired to detectthese electrons individually to get detailed information on the objectto be examined.

[0016] A technology for detecting these electrons individually has beendisclosed in Japanese Non-examined Patent Publication No.07-192679(1995). This technology deflects the direction of motion of thesecondary electrons by a deflector, separates the secondary electronsfrom the reflected electrons, and detects the secondary electrons andthe reflected electrons individually by corresponding detectors whichare placed in the moving passages of the electrons.

[0017] However, the above-mentioned technology combined with theretarding technology will allow the secondary electrons to have as highenergy as that of the reflected electrons and consequently, their movingtracks become almost the same and they cannot be separated from eachother.

[0018] As explained above, it is apparent that any conventionaltechnology disclosed in the above patent specifications is hard todetect the secondary electrons and the reflected electrons individuallywhen combined with the retarding technology.

[0019] An object of the present invention is to provide a scanningelectron microscope (SEM) employing a retarding technology which reducescharge-up of the specimen and accomplishes low chromatic aberration andhigh resolving power and capable of detecting secondary electronsindependently of the other electrons.

[0020] The object of the present invention is accomplished by a scanningelectron microscope consisting of an electron source, a lens forcondensing a primary electron beam emitted from said electron source,detectors for detecting electrons which are generated by radiation ofthe primary electron beam condensed by said lens onto a specimen, afirst decelerating means for decelerating the primary electron beambefore the primary electron beam hits said specimen, a seconddecelerating means for decelerating the electrons generated by collisionof electrons against the specimen, and deflectors for deflecting theelectrons decelerated by said second decelerating means to saiddetectors.

[0021] The scanning electron microscope of the aforesaid configuration,even when it employs the retarding technology, can selectively detectelectrons of lower energies among those generated by radiation of theprimary electron beam (e.g. secondary electrons).

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic block diagram showing a scanning electronmicroscope which is an embodiment of the present invention.

[0023]FIG. 2 is a schematic block diagram showing the configuration ofthe first detector which deflects secondary electrons away from theaxis.

[0024]FIG. 3 is a schematic block diagram explaining the configurationwhich selectively detects secondary electrons of lower energies.

[0025]FIG. 4 is a schematic block diagram showing the configurationwhich selectively detects secondary electrons of lower energies.

[0026]FIG. 5 is a schematic block diagram showing a scanning electronmicroscope which is another embodiment of the present invention.

[0027]FIG. 6 is a whole schematic block diagram showing a scanningelectron microscope which is another embodiment of the presentinvention.

[0028]FIG. 7 is a schematic block diagram explaining the principle ofdetection of secondary electrons in the scanning electron microscope ofFIG. 1.

[0029]FIG. 8 is a schematic block diagram explaining the principle ofdetection of secondary electrons in the scanning electron microscope ofFIG. 6.

[0030]FIG. 9 is a horizontal sectional view of the secondary electronbeam detector of a scanning electron microscope which is an embodimentof the present invention, explaining the structure of an electrode fordeflecting secondary electrons towards the scintillator.

[0031]FIG. 10 is a vertical sectional view of the secondary electronbeam detector of a scanning electron microscope which is an embodimentof the present invention, explaining the structure of an electrode forconcentrating secondary electrons to the center of the scintillator.

[0032]FIG. 11 is another vertical sectional view of the secondaryelectron beam detector of a scanning electron microscope which is anembodiment of the present invention, explaining the structure of anelectrode for concentrating secondary electrons to the center of thescintillator.

[0033]FIG. 12 is a schematic block diagram of the second signal detectorwhich uses a Butler type electrode in the area to decelerate primaryelectrons.

[0034]FIG. 13 is a schematic block diagram explaining how the charge-upof a specimen causes a problem (detection of no secondary electrons).

[0035]FIG. 14 is a schematic block diagram showing a low accelerationvoltage scanning electron microscope in accordance with the presentinvention which employs the retarding technology and acceleration at alower stage.

[0036]FIG. 15 is a schematic block diagram explaining a method ofsolving a problem of FIG. 13 by a surface correction voltage.

[0037]FIG. 16 is a graph showing the relationship between the surfacecorrection voltage and the output of the secondary electron multipliertube.

[0038]FIG. 17 is a schematic diagram explaining a circuit whichautomatically controls the intensity and the amplitude of the output ofthe secondary electron multiplier tube.

[0039]FIG. 18 is a graph showing the relationship between the surfacecorrection voltage and the output of the secondary electron multipliertube when the scanning electron microscope has a circuit of FIG. 17.

[0040]FIG. 19 is a schematic block diagram explaining another means toform a deceleration field.

[0041]FIG. 20 is a schematic block diagram explaining another means toform a deceleration field.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0042] The embodiments of the present invention will be explained belowin reference with the accompanying drawings. All the embodiments are ofa scanning electron microscopes employing a retarding technology tosuppress charge-up of a specimen and accomplish low chromatic aberrationand high resolving power by forming a deceleration field to deceleratethe motion of electrons accelerated by retarding and detecting secondaryelectrons (signal) decelerated by said deceleration field.

[0043] [Embodiment 1]

[0044]FIG. 1 is a schematic block diagram of a scanning electronmicroscope which is an embodiment of the present invention. When anextraction voltage 3 is applied between an electron source (electronemitting cathode 1) and the extraction electrode 2, electrons 4 areemitted from the cathode 1. The electrons 4 are accelerated (ordecelerated in some cases) between the extraction electrode 2 and theanode 5 at the grounding voltage. When passing through the apertures ofanode 5, the electron beam (primary electron beam 7) undergoesacceleration by the electron gun acceleration voltage 6.

[0045] The primary electron beam 7 accelerated by the anode undergoesscanning deflection by the condenser lens 14, the upper scanningdeflector 15 and the lower scanning deflector 16. The upper and lowerdeflectors work to deflect the electron beam around the center axis ofthe object lens 17 to scan the object 12 to be examined (a specimen) ina 2-dimensional manner. The deflected primary electron beam 7 furtherundergoes acceleration by an acceleration voltage 22 from theacceleration tube 9 in the passage of the object lens 17.

[0046] The accelerated primary electron beam 7 is finely concentratedonto the specimen 12 by the object lens 17. A deceleration field of anegative voltage (hereinafter called a retarding voltage 13) which isapplied via the specimen holder 100 is formed between the object lens 17and the specimen 12. The primary electron beam 7 passing through theobject lens 17 is decelerated in this deceleration field before reachingthe specimen 12.

[0047] The aperture 8 controls the opening angle of the primary electronbeam 7 and it can be center-aligned by the control 10. The mechanism 19for moving the specimen 12 in the X and Y directions has a specimenholder 100 insulated with an insulating plate 21 on it. A retardingvoltage 13 is applied to the specimen holder 100. A specimen (e.g. awafer) is placed on the holder 100. Naturally, the retarding voltage 13is also applied to the specimen 12 on the holder.

[0048] In this configuration, the acceleration voltage (sum of theelectron gun acceleration voltage 6 and the acceleration voltage 22 atthe lower stage) applied to the primary electron beam 7 which passesthrough the object lens 17 is higher than the acceleration voltageapplied to the electrons which hit the specimen (=the electron gunacceleration voltage 6 minus the retarding voltage 13). Consequently,this configuration can get a more-concentrated electron beam (of higherresolving power) than a primary electron beam concentrated by the objectlens 17.

[0049] This is because the chromatic aberration of the object lens 17reduces. In a typical example applying an electron gun accelerationvoltage 6 of 2 kV, an acceleration voltage 22 at the lower stage of 7kV, and a retarding voltage of 1 kV, the primary electron beam 7 passesthrough the object lens 17 with a voltage of 9 kV and hits the specimenwith an acceleration voltage 13 of 1 kV. The resolving power of thisexample is 3 nm, which is about on third of that (10 nm) ofmagnification at the acceleration voltage of 1 kV.

[0050] When the primary electron beam 7 irradiates the specimen 12 asecondary signal 23 generates. The secondary signal 23 mainly consistsof secondary electrons and reflected electrons.

[0051] The electric field formed between the object lens 17 and thespecimen 12 works to accelerate the generated secondary signal 23. Thesecondary signal is attracted into the passage in the object lens 17 andmoved up while undergoing the lens action by the magnetic field of theobject lens 17. The secondary signal 23 passing through the object lens17 passes by the scanning deflectors 15 and 16.

[0052] In the scanning electron microscope disclosed as an embodiment ofthe present invention, the secondary signal 23 passing through thescanning deflectors are decelerated by the deceleration electric fieldformed between the electrostatic deflectors 41 a and 41 b.

[0053] A retarding voltage 13 is applied to the specimen 12 and to amid-point between the deflection voltages (negative deflection voltage46 and positive deflection voltage 47) which are applied to theseelectrostatic deflectors 41 a and 41 b.

[0054] Below will be explained the principle of detection of thesecondary signal 23 in this embodiment. This example uses an electrongun acceleration voltage 6 of 2 kV, an acceleration voltage 22 at thelower stage of 7 kV, and a retarding voltage 13 of 1 kV, however thesevalues are intended to explain the invention and are not to be construedto limit the scope of the invention.

[0055] The secondary signal 23 generated on the specimen 12 (whichconsists of secondary electrons whose emission energy is in the range of0 V to about 10 V and the highest at about 2 V and reflected electronswhose emission energy is 1 kV) is accelerated by the retarding voltage13 which is applied to the specimen 12 and by the acceleration voltage22 at the lower stage. With this, the secondary electrons having energyof 8 kV and the reflected electrons having energy of 9 kV pass throughthe object lens 17.

[0056] After passing by the acceleration electrode 9 at the lower stage,the secondary signal 23 is decelerated by 7 kv. The secondary electronsare decelerated to have an energy of 1 kV and the reflected electronsare decelerated to have an energy of 2 kV.

[0057] As the retarding voltage 13 is applied to a mid-point between theelectrostatic deflection electrodes 41 a and 41 b of the lower detector,the secondary and reflected electrons are further decelerated in thespace formed by the electrostatic deflectors 41 a and 41 b to have theinitial energies (0 to about 10 V for the secondary electrons and about1 kV for the reflected electrons). Namely their energies are decrementedby the retarding voltage 13.

[0058] The field made by electrostatic deflection electrodes 41 a and 41b deflects only the secondary electrons 50 having low energy to make thesecondary electrons pass through the electrostatic deflection electrode41 b (wire grid). The secondary electrons hit the scintillator toilluminate it. This illumination is guided by the light guide into thephoto-electron multiplier tube 45, converted into an electric signal,and amplified. Only the secondary electrons having lower energy aredetected here. The magnetic deflection coils 40 a and 40 b work tocorrect the deflection of the primary electron beam 7 by theelectrostatic deflection electrodes 41 a and 41 b.

[0059] The reflected electrons 51 still has energy of about 1 kV afterthey undergo deceleration in the deceleration field formed by theelectrostatic deflection electrodes 41 a and 41 b. Accordingly, thereflected electrons 51 pass by the first detector almost without beingdeflected in the field made by the electrostatic deflection electrodes41 a and 41 b. The reflected electrons 51 further undergo accelerationby the retarding voltage 13, enter the second detector 35 with energy ofabout 2 kV, and hit the reflecting plate 29. The secondary electrons 30reflected on the plate 29 are deflected by the electrostatic deflectionelectrodes 31 a and 31 b, driven through the electrostatic deflectionelectrode 31 b (wire grid), and detected. The electrostatic deflectionelectrode 31 b is a wire grid through which the deflected secondaryelectrons can pass. The magnetic deflection coils 33 a and 33 b generatea magnetic field perpendicular to the electric field generated by theelectrostatic deflection electrodes 31 a and 31 b to cancel the staticdeflection on the primary electron beam 7. The signal detected herecontains information of the reflected electrons 51.

[0060] The reflecting plate 29 is a conductive plate having a centeraperture through which the primary electron beam 7 can pass. Its surfaceagainst which the reflected electrons hit is covered with a materialwhich generates secondary electrons efficiently (e.g. deposited withgold).

[0061] The secondary electrons passing through the electrostaticdeflection electrode 31 b (wire grid) are attracted by the scintillator32 to which a positive high voltage (10 kV) is applied. Then thesecondary electrons hit the scintillator to illuminate it. Thisillumination is guided by the light guide 24 into the photo-electronmultiplier tube 18, converted into an electric signal, and amplified.The output of the photo-electron multiplier tube 18 is used forintensity modulation of the CRT tube (not visible in the drawing).

[0062] The aforesaid configuration can detect secondary electrons 50 bythe first detector and secondary electrons containing information of thereflected electrons 51 by the second detector 35. The reflected plate 29can be substituted by a channel plate detector to directly detectelectrons reflected on the plate.

[0063] The conventional scanning electron microscope having a reflectingplate can detect both reflected and secondary electrons but cannotdistinguish secondary electrons from reflected electrons. For example,in a case if creating a sample image by reflected electrons only, theinformation of the reflected electrons is affected by information ofsecondary electrons and consequently the image creation fails.

[0064] Similarly, the conventional scanning electron microscope having achannel-plate detector can detect reflected electrons only or bothsecondary electrons and reflected electrons by controlling an energyfilter placed between the channel plate and a specimen, but cannotdetect the reflected electrons and the secondary electrons individually.For example, to respectively obtain a sample image by reflectedelectrons and a sample image by secondary electrons, the scanningelectron microscope must perform an electron beam scanning to form areflected electron image and an electron beam scanning to form asecondary electron image (affected by reflected electrons), whichreduces the through-put, requires a long-time radiation of an electronbeam, and consequently causes charge-up of the specimen.

[0065] In some cases, the reflected electrons may cancel a contrast madeby secondary electrons. So there has been wanted a scanning electronmicroscope capable of detecting secondary electrons which are notaffected by reflected electrons.

[0066] The scanning electron microscope in accordance with the presentinvention can solve all of the aforesaid problems. Even when a secondarysignal is accelerated by the retarding technology, said scanningelectron microscope can separate secondary electrons from reflectedelectrons and detect the reflected electrons only or both the reflectedelectrons and the secondary electrons simultaneously.

[0067] As the reflected electrons has more excellent linearity andpermeability than the secondary electrons, the reflected electrons arefit to observe the bottoms of contact holes formed in semiconductordevices.

[0068] Further as the reflected electrons can make the image contrasthigher, they can be used, for example, to check semiconductor devicesfor resist residue and to clearly detect an alignment mark whosematerial is different from semiconductor device materials for exactpositioning and alignment.

[0069] Meanwhile, the secondary electrons have more signals than thereflected electrons and are fit to observe the fine structure of acontact hole and its vicinity on the sample surface. The scanningelectron microscope in accordance with the present invention enablesobservation of both the bottom and vicinity of a contact hole and candisplay an image formed by electrons detected by the first detector 34(“Sample surface image”) and an image formed by electrons detected bythe second detector 35 (“Contact hole image”) on the display unit of themicroscope (not visible in the drawing).

[0070] Further the scanning electron microscope in accordance with thepresent invention can combine the reflected electron image and thesecondary electron image into a composite image by varying the ratio ofsignals output from the photo-electron multiplier tubes 18 and 45. Inthis case, a composite image can be formed from information of thereflected electrons and the secondary electrons which are emitted froman identical object to be examined as the reflected electrons and thesecondary electrons can be detected simultaneously. Furthermore, thescanning electron microscope in accordance with the present inventioncan detect the secondary electrons and the reflected electronsindividually, forms their sample images separately, and stores theimages in frame memory (not visible in the drawing). So the ratio of thesignals can be varied freely. Therefore, the electron beam need not beemitted onto the specimen so long to vary the ratio of signals tocompose an image. The images further undergo proper processing into afinal image.

[0071] However the above examples of explicitly detecting secondary andreflected electrons are intended to explain the invention and are not tobe construed to limit the scope of the invention. For example, in thecase the retarding voltage applied to the electrostatic deflectionelectrodes 41 a and 41 b is made a little lower than the retardingvoltage applied to the specimen, a little amount of secondary electronstogether with the reflected electrons 51 are caused to hit thereflecting plate 29, converted into secondary electrons and detected bythe second detector. In other words, the configuration can be modifiedto cause the first detector 34 to detect secondary electrons havinglower energy (acceleration voltage) and the second detector 35 to detectsecondary electrons having higher energy.

[0072] In this detecting method, the sample image formed by electronsdetected by the second detector 35 contains more information about thereflected electrons than the image formed by electrons detected by thefirst detector 34. For example, by using the first detector 34 toobserve a semiconductor area including contact holes or resist residuesand the second detector 35 to observe conductor patterns on a specimenor measure dimensions of patterns, observation modes (Contact Hole mode,Pattern mode, etc.) can be switched without any complicated control.

[0073] In the above explanation, this embodiment is assumed to have anacceleration tube 9, but it is apparent that the same effect can beobtained without the tube 9. Similarly, the same effect can be obtainedwhen a retarding voltage 13 is not applied to the sample. Namely, bothof provision of the acceleration tube 9 and application of a retardingvoltage to a sample are not required to obtain the aforesaid effect. Thescanning electron microscope in accordance with the present inventioncan have provision of the acceleration tube 9, application of aretarding voltage to a sample, or none. Further there have been manyother technologies to form a deceleration field. They will be explainedbelow.

[0074] [Embodiment 2]

[0075]FIG. 2 shows the horizontal sectional view of the first detectorwhich is perpendicular to the motion of a primary electron beam. Theright half 41 b of the cylindrical static deflector 41 a and 41 b isslitted longitudinally so that the deflected electrons 50 can passthrough the right half 41 b of the static deflector. Th electric field Efor deflecting secondary electrons 50 is generated by a negativedeflecting voltage 46 and a positive deflecting voltage 47. Usually,these positive and negative deflecting voltages 46 and 47 are of thesame magnitude. A retarding voltage is applied to their mid-point.

[0076] The deflection coils 40 a and 40 b produce a deflecting magneticfield B which perpendicularly intersect the electric field E. Themagnitude and direction of this magnetic field can be controlled tocancel the deflection of the primary electron beam by the deflectionfield E.

[0077] Contrarily, this magnetic field works to deflect secondaryelectrons 50 further. A filtering wire grid 42 is placed outside thestatic deflector 41 b along its surface to separate secondary electronsby a difference of emission energies. A filter voltage 48 is applied toseparate secondary electrons to the mid-point of the positive andnegative deflecting voltages 46 and 47 (to which a retarding voltage isapplied).

[0078] When a positive voltage is applied to the filtering wire grid 42,the filtering wire grid 42 allows secondary electrons in the wholeenergy range. When a negative voltage is applied to the filtering wiregrid 42, the filtering wire grid repulses secondary electrons of energyequivalent to the negative voltage and lower. The secondary electronspassing through the wire grid 42 are attracted and accelerated by thescintillator 43 to which 10 kV is applied, and hits the scintillator 43to illuminate. The illumination is guided by the light guide 44.

[0079] It is also possible to turn off the retarding voltage 13 which isadded to the mid-point of the static deflectors 41 a and 41 b in FIG. 1to drive up the secondary electrons and cause the second detector 35 todetect both secondary and reflected electrons.

[0080] [Embodiment 3]

[0081] The embodiment of FIG. 1 applies a retarding voltage to themid-point of the static deflectors 41 a and 41 b of the first detector34. It is also possible to selectively detect secondary electrons of aspecific energy by adding a voltage to the retarding voltage. Referringto FIG. 3, the principle of the detection will be explained below.

[0082] The configuration in FIG. 3 can selectively detect secondaryelectrons of a lower energy. This configuration has an upper wire grid54 above the cylindrical static deflector 41 a and 41 b and a lower wiregrid 55 below the static deflectors 41 a and 41 b. These wire grids 54and 55 are connected to the mid-point of the deflection voltages 46 and47 of the static deflectors 41 a and 41 b. This example applies apositive voltage such as 5 V to a superposition voltage 53.

[0083] In this status, the sum of the retarding voltage (negative) andthe superposition voltage 53 (positive) is applied to the mid-point ofthe static deflectors 41 a and 41 b. This makes secondary electrons inthe cylindrical static deflector 41 a and. 41 b have higher energy (by 5V) than the secondary electrons in FIG. 2. Therefore, secondaryelectrons having higher emission energy undergo less deflection and passby the detector without being detected. As seen from this example,secondary electrons having lower emission energy can be selectivelydetected by adding a positive voltage to the retarding voltage.

[0084] The upper and lower wire grids 54 and 55 in this embodiment canbe omitted but the same effect can be obtained. The upper and lower wiregrids 54 and 55 respectively have an aperture through which the primaryelectron beam passes.

[0085] When the second detector 35 is placed in the movement of thesecondary electrons of higher energy, secondary electrons of lowerenergy can be detected by the first detector 34 and secondary electronsof higher energy can be detected by the second detector 35. Thereflecting plate 29 should be removed to cause the second detector todetect only secondary electrons of higher energy including no reflectedelectrons. For this purpose, the reflecting plate should be insertablefrom the outside of the axis of the electron beam according toconditions of measurement. The position of the detector and theprovision of the reflecting plate in this embodiment can be setaccording to specimen compositions and observation modes.

[0086] [Embodiment 4]

[0087]FIG. 4 shows an example of selectively detecting secondaryelectrons of higher energy. FIG. 4 uses a negative superposition voltage53 (unlike the example in FIG. 3). The negatively-charged lower wiregrid 55 repulses secondary electrons of energy equivalent to thenegative voltage and lower. Therefore, only the secondary electrons ofhigher energy that passed through the wire grid 55 can be detected. Inthis example, (as the second detector 35 equipped with the reflectingplate 29 can detect reflected electrons) the first detector 34 detectssecondary electrons of higher energy and the second detector 35 detectsreflected electrons.

[0088] The scanning electron microscope in accordance with the presentinvention need not produce a great deflection field because theaccelerated secondary electrons are decelerated by a retarding voltageand detected by a detector which is placed away from the beam axis. Inthis status, secondary electrons can be selectively guided to the staticdeflection electrode 41 b by producing a little potential differencebetween the electrostatic deflection electrodes 41 a and 41 b.

[0089] This configuration can form a deceleration field between thespecimen 24 and the static deflection electrodes 41 and 42 which is asstrong as the acceleration field (viewed from the secondary signal)produced between the specimen 24 and the object lens 17. Therefore, thesecondary electrons and the reflected electrons which pass through thestatic deflection electrodes 41 and 42 behave as if retarding is notperformed.

[0090] It is recommended that the deflection field produced between thestatic deflection electrodes 41 a and 41 b should be as weak aspossible. If this deflection field is strong, it may deflect not onlythe secondary signal but also the primary electron beam and consequentlyit may cause an off-axis aberration.

[0091] Although the embodiment of the present invention has a means toproduce a magnetic field which perpendicularly intersects the deflectionfield formed by the static deflector to push back the primary electronbeam deflected by the static deflector. (Orthogonal magnetic fieldgenerator: disclosed in Japanese Non-examined Patent PublicationNo.09-171791 (1997)) If the static deflector has a great deflectingforce, the energy of the primary electron beam is dispersed and anaberration may occur. Accordingly, it is recommended to make thedeflection field as weak as possible even when the orthogonal magneticfield generator is employed.

[0092] [Embodiment 5]

[0093]FIG. 5 shows the configuration of a scanning electron microscopewhich is another embodiment of the present invention. In thisembodiment, the magnetic path of the object lens 17 is divided into anupper magnetic path 25 and a lower magnetic path 26. A loweracceleration voltage 22 is applied to the upper magnetic path and worksas a lower acceleration electrode. This configuration reduces apossibility of misalignment of the lower acceleration electrode to theaxis of the object lens in comparison to a lower acceleration electrodewhich is provided separately.

[0094] This embodiment has a control electrode 27 between the specimen12 and the object lens 17′. A voltage applied to the specimen 12 is alsoapplied to this control electrode 27. This configuration prevents thesurface of the insulating specimen from electrically floating in theelectric field produced between the specimen 12 and the object lens 17.This control electrode 27 can give the specimen the same voltage as thatapplied to the specimen holder 100.

[0095] The reflecting plate 29 of this embodiment is tilted towards thescintillator 32 to increase the generation of secondary electronsefficiently by the collision of reflected electrons.

[0096] [Embodiment 6]

[0097]FIG. 6 shows the configuration of a scanning electron microscopewhich is another embodiment of the present invention. Component numbersin FIG. 6 are the same as those in FIG. 1 Components of new numbers willbe explained below. The liner tube 106 is placed on the inside (on theaxis side) of the upper and lower scanning deflectors 15 and 16. Theliner tube 106 is grounded to have a ground potential.

[0098] This embodiment has static deflection electrodes 101 a and 101 binstead of the static deflection electrodes 41 a and 41 b of the firstdetector 34. The static deflection electrodes 110 a and 101 b aretapered towards the top. Below will be explained the difference betweenthe static deflection electrodes 111 a and 101 b of this embodiment andthose 41 a and 41 b of the first embodiment.

[0099]FIG. 7(A) shows the schematic block diagram of the electrostaticdeflection electrodes 41 a and 41 b and their vicinity in FIG. 1. FIG.7(B) shows the distribution of potentials along the axis. The secondaryelectrons 102 which are accelerated by the retarding voltage (notvisible in the drawing) applied to the specimen are decelerated in thesecondary electron detecting unit 103. A voltage almost equivalent tothat applied to the specimen is applied to this secondary electrondetecting unit 103 and the secondary electrons 102 are decelerated downto energy of about 10 eV in the acceleration/deceleration area 104.

[0100] The decelerated secondary electrons 102 are deflected by thestatic deflection electrode 41 a to which a negative voltage is applied(relative to the specimen voltage) and the static deflection electrode41 b to which a positive voltage is applied. The static deflectionelectrode 41 b is a wire grid through which the deflected secondaryelectrons 5 can pass through. The magnetic deflection coil 40 a producesa magnetic field which perpendicularly intersects an electric field thatthe static deflection electrodes 41 a and 41 b produce to canceldeflection of the primary electron beam by the static deflectionelectrodes.

[0101] The secondary electrons 5 passing through the static deflectionelectrode 41 b (wire grid) are attracted to the scintillator 43 to whicha positive high voltage (10 kV) is applied and hit the scintillator 43to illuminate it.

[0102] When the retarding voltage 13 is low (e.g. a few hundred voltagesor lower), the secondary electrons 102 are efficiently deflected towardsthe static deflection electrode 41 b by a deflection field which isproduced by the static deflection electrodes 41 a and 41 b. When theretarding voltage 13 goes higher (e.g. a few hundred voltages orhigher), the deceleration field penetrating into the space between thestatic deflection electrodes 41 a and 41 b from the entrance area 104becomes stronger than the deflection field produced by the staticdeflection electrodes 41 a and 41 b and consequently, secondaryelectrons 102 may be repulsed back towards the specimen 12.

[0103] This phenomenon is more striking when the distance between thestatic deflection electrodes 41 a and 41 b is made wider (to have alarger diameter) to take in secondary electrons 5 which are deflectedmuch away from the axis of the electron beam by the scanning deflectors15 and 16.

[0104] This kind of field penetration also occurs in theacceleration/deceleration area 105. In some cases, the secondaryelectrons 102 entering the space between the static deflection electrode41 a and 41 b may pass through the deflection area and theacceleration/deceleration area 105.

[0105] To solve the aforesaid problems, this embodiment has staticdeflection electrodes 110 a and 101 b which are tapered towards thesource of electrons. FIG. 8 shows the details of the static deflectionelectrodes. FIG. 8(A) shows the schematic block diagram of the staticdeflection electrodes 110 a and 101 b which are tapered towards thesource of electrons and their vicinity. FIG. 8(B) shows the distributionof potentials along the axis.

[0106] The lower ends of the static deflection electrodes 101 a and 101b which produce a traverse electric field are wide-spaced to catchsecondary electrons 5 which move further away from the axis of theelectron beam. The uppers part of the static deflection electrodes 101 aand 101 b are made narrower and the resulting deflection field becomesstronger than that of the lower half of the static deflection electrodes101 a and 101 b. The secondary electrons 5 taken into the space of thestatic deflection electrodes 101 a and 101 b are efficiently deflectedtowards the static deflection electrode 101 b by the comparativelystrong deflection field produced by the upper half of the staticdeflection electrodes 101 a and 101 b.

[0107] This electrode configuration enables capture of almost allsecondary electrons including those going further away from the axis ofthe electron beam and high-efficient deflection of the capturedelectrons to the detector.

[0108] As already explained, the potential of the area which deflectssecondary electrons is approximately equal to the potential of thespecimen. This potential causes the acceleration/deceleration areas 107(entrance) and 105 (exit) to produce a strong deceleration field and astrong acceleration field respectively. If these strong electric fieldsinvade the deflection space formed by the static deflection electrodes101 a and 101 b, they may repulse the secondary electrons back to thespecimen as already explained. To prevent this problem (or to reduce theinfluence of the electric field upon the space formed by the staticdeflection electrodes 101 a and 101 b), the embodiment of the presentinvention has a deceleration electrode 108 whose voltage can becontrolled independently of the retarding voltage and the boostingvoltage in the acceleration/deceleration area 107.

[0109] This intermediate deceleration electrode 108 decelerates thesecondary electrons 102 coming from the specimen in multiple stages (2stages 107 a and 107 b in this embodiment). Therefore, this intermediatedeceleration electrode 108 can reduce the influence of the electricfield upon the space formed by the static deflection electrodes 110 aand 101 b.

[0110] Further, a grid 109 on the deceleration electrode 109 can controlpenetration of the deceleration field more effectively. The grid 109 hasan aperture in the center (where the axis of the electron beamintersects) to allow the primary electron beam 110 to pass through.

[0111] In the above explanation, the embodiment has the staticdeflection electrodes 101 a and 101 b which are tapered towards thesource of electrons. This electrode structure can be substituted by atapered cone-like electrodes which are splitted longitudinally or a unitconsisting of two flat electrode plates which come close to each otheras they go up to the source of the electrons. Further the number of suchelectrodes can be two or more.

[0112] The liner tube 106 need not always be in the ground potentiallevel. For example, it can be used as an acceleration cylinder. However,this embodiment grounds the liner tube 106 to control penetration of thedeceleration field into the deflection area.

[0113] [Embodiment 7]

[0114]FIG. 9 shows the schematic block diagram of another example ofmeans for forming an electric field to reduce secondary signals.

[0115]FIG. 9 shows the horizontal sectional view of the secondaryelectron detector which is perpendicular to the motion of a primaryelectron beam. The static deflection electrodes 101 a and 101 b areconstructed to form a circular truncated cone whose side surface issplitted along the slant (which is not visible in the drawing). Thestatic deflection electrode 101 b is made in a grid form to letsecondary electrons pass through it.

[0116] The electric field E for deflecting secondary electrons 102 isproduced by the negative deflection voltage 46 and the positivedeflection voltage 47 which is increased by the retarding voltage 13.The magnitude and direction of this magnetic field can be controlled tocancel the deflection of the primary electron beam by the deflectionfield E. This magnetic field works to deflect secondary electrons 102further.

[0117] The deflection coil 11 consists of four coil elements which arecontrolled independently to control the direction and strength ofdeflection by the deflection magnetic field B. This configuration cancorrect the perpendicularity of the electric field E to the magneticfield B.

[0118] The static deflection electrodes 101 a and 101 b are surroundedby a reflective wall electrode 112 to guide the secondary electrons 102which comes through the grid of the static deflection electrode 101 b tothe scintillator 43.

[0119] The reflective wall electrode 112 and the static deflectionelectrode 101 a can be built in a body as these electrodes can be in thesame potential level. The secondary electrons 102 passing through thestatic deflection electrode 101 b are attracted to the scintillator 43to which 10 kV is applied, accelerated and finally hit the scintillator43 to illuminate.

[0120] In this embodiment, as a retarding voltage 13 is applied to thestatic deflection electrodes 101 a and 101 b, a potential differencegenerates between the electrodes 101 a and 101 b and the body tube (notvisible in the drawing) and this potential has a possibility to scatterthe secondary signal coming up from the specimen. This problem can besolved by surrounding the static deflection electrodes 101 a and 101 bby a reflective wall electrode 112.

[0121] [Embodiment 8]

[0122]FIG. 10 and FIG. 11 show how the secondary electrons 102 passingthrough the static deflection electrode 101 b are guided to thescintillator 43 by the cylindrical electrode 113 for collectingsecondary electrons. To deflect secondary electrons 102 coming throughthe whole grid of the static deflection electrode 101 b efficiently tothe scintillator which has a narrower effective area than the wire grid,the cylindrical electrode 113 for collecting secondary electrons isused.

[0123]FIG. 10 shows a method of concentrating the secondary electrons102 passing through the static deflection electrode 101 b directlytowards the scintillator 43. When a retarding voltage 13 is applied tothe cylindrical electrode 113, the secondary electrons 102 undergoacceleration by an acceleration electric field of the grounded outercylinder 114 of the scintillator 43 and jump into the scintillator 43which is placed in the center of the cylindrical electrode 113.

[0124] Meanwhile, FIG. 11 shows a method of accelerating the secondaryelectrons 102 passing through the static deflection electrode 101 b tomove straight on. The accelerated secondary electrons hit the innersurface of the cylindrical electrode 113 or the outer cylinder 114 ofthe scintillator 43 and generate new secondary electrons 115.

[0125] A wire grid 116 attached to the opening of the cylindricalelectrode 113 which is faced to the static deflection electrode 101 bprevents the electric field produced by the static deflection electrode101 b from invading the inside of the cylindrical electrode 113. Thiscauses the capturing electric field from the scintillator 43 to which avoltage 117 of about 10 kV is applied to penetrate into the cylindricalelectrode 113. As the result, secondary electrons 115 are captured bythe scintillator 43 efficiently.

[0126] [Embodiment 9]

[0127]FIG. 12 shows an embodiment which applies a retarding voltage 13to both a unit 103 for detecting secondary electrons and a unit 118 fordetecting reflected electrons. The primary electron beam 110 undergoesdeceleration on passing through the Butler type deceleration lens 119,goes through the reflected electron detector 118 and the secondaryelectron detector 103, and undergoes acceleration in the space of thedeceleration electrode 108.

[0128] As the retarding voltage 13 (1 kV) is comparatively greatrelative to the acceleration voltage of the electro gun (2 kV) and thereflecting plate 29 has a very small opening (about 1 mm in diameter) tolet the primary electron beam 110 pass through, an aberration is apt togenerate and consequently the diameter of the primary electron beam mayincrease. However the Butler type deceleration lens, when installed, candecelerate the primary electron beam while suppressing the generation ofan aberration. The reflected electron detector 118 and the secondaryelectron detector 103 are isolated from each other by a wire gridelectrode 120. A negative voltage 122 which is higher by a few tenvoltages than the deflection electrode voltage 121 is applied to thewire grid electrode 120, which prevents the secondary electrons 102 fromgoing into the reflected electron detector 118. This also prevents thesecondary electrons 123 reflected by the reflecting plate 29 from goinginto the secondary electron detector 103.

[0129] After passing through the grid-like static deflection electrode101 b by the deflection force produced by the static deflectionelectrodes 101 a and 101 b, the secondary electrons 102 are attracted tothe scintillator 43 to which a positive high voltage of 10 kV is appliedand hit the scintillator 43 to illuminate. The light of illumination isguided by the light guide into the photo-electron multiplier 45,converted into an electric signal and amplified there. This output isused to perform the brightness modulation of a CRT tube (not visible inthe drawing).

[0130] Meanwhile, the reflected electron detector 118 causes thereflected electrons 124 passing through the secondary electron detector103 to be reflected by the reflecting plate 29 and guides to thescintillator 32. In other words, when the reflected electrons 124 hitthe reflecting plate 29, new secondary electrons 123 come out from thereflecting plate, undergo deflection by the deflection field produced bythe static deflection electrodes 31 a and 31 b, and pass through thewire grid of the static deflection electrode 31 b. The succeedingoperation of the reflected electron detector 118 is the same as that ofthe secondary electron detector 103.

[0131] [Embodiment 10]

[0132] A charge-up of a specimen in observation through an electronmicroscope disturbs the motion of secondary and reflected electrons fromthe specimen and consequently causes extraordinary contrasts anddistortions in the obtained images.

[0133] Although the retarding technology suppresses charge-up of aspecimen by using primary electron beams of low energy, the specimencannot be free from being charged up as the low energy of the primaryelectron beam is strong enough to charge up the specimen.

[0134] The scanning electron microscope in accordance with the presentinvention has a function to apply a voltage to eliminate the charge ofthe specimen. The detailed means will be explained below.

[0135] The present invention will be more clearly understood withreference to a configuration of a scanning electron microscope whichdoes not use the principle of this embodiment.

[0136] The explanation below assumes that the low-acceleration scanningelectron microscope will observe a semiconductor specimen having aresist or silicon oxide film pattern on an insulation layer formed on asilicon wafer. In this case, the specimen is charged up. This charge issteady at a certain positive voltage depending upon an accelerationvoltage or the like without changing as the time goes by. However, thispositive voltage value is not known.

[0137] For example, the positive voltage on a resist pattern formed on asilicon wafer is about a few voltages, but the positive voltage on aresist pattern on a silicon oxide layer formed on a silicon wafer ishigher than 10 V. If the latter specimen is observed by a scanningelectron microscope of FIG. 1, it sometimes happens that the firstdetector 34 detects no signal or less signals than expected. Thisproblem not only reduces the signals but also changes the magnificationof a scanning image. In semiconductor processes, exact measurement ofdimensions of patterns on semiconductor wafers is very significant anddimensional errors due to charge-up of specimens cannot be ignored.

[0138] Referring to FIG. 13, this problem will be explained in detail.FIG. 13 contains only the specimen section 12 and the first detectorsection 34 that are required for explanation. Components that are alsofound in FIG. 1 will not be explained here.

[0139] The specimen 12 contains a pattern 62 of insulating materialformed on a silicone wafer 61. This pattern 62 is positively charged byexposure to the primary electron beam 7. The magnitude (in voltages) ofthis charge-up is dependent upon the energy of the primary electron beam7 applied to the specimen 12.

[0140] The first detector 34 has a wire grid 55 to which a retardingvoltage 13 is applied at its entrance. The secondary electrons 50emitted from the specimen 12 at the same potential as the retardingvoltage 53 can go into the first detector 34 but the positively-chargedsecondary electrons 50 (in FIG. 13) are repulsed by the wire grid 55 andcannot get to the first detector 34.

[0141]FIG. 14 shows the configuration of an embodiment of the presentinvention to solve such a problem. FIG. 1 and FIG. 14 are the sameexcept that FIG. 1 has a surface voltage correcting voltage source 124(means for applying a variable voltage) between the specimen and theretarding voltage 53.

[0142] As disclosed in FIG. 15, the surface voltage correcting voltagesource 63 provided between the specimen 12 and the retarding voltage 53is controlled to apply such a voltage that eliminates the charge of thespecimen 12. With this, an optimum image is obtained.

[0143] The operator can manually control this voltage while observingthe obtained image on the monitor screen (of the CRT or the like). It isvery hard to know how much the specimen is charged singly from theobtained specimen image, but the contrast of the obtained specimen imagevaries depending upon the quantity of secondary electrons detected bythe detector. Therefore, the operator has only to control the surfacecorrection voltage to get a specimen image of the optimum contrast.

[0144] When a specimen 12 covered with an insulating film is observed bya scanning electron microscope, the first detector 34 may fail to detecta signal due to the charge-up of the specimen. Contrarily, the seconddetector 35 can observe the specimen image independently of whether ornot the specimen is charged up as the detector 35 detects the reflectedelectrons. Accordingly, the second detector 35 can be used to check thelocation of a specimen to be examined.

[0145] [Embodiment 11]

[0146] Below will be explained another example to set a surface voltagecorrecting voltage 124. The first step of this method is to graduallyincrease the correction voltage 124 while monitoring the output of thesecondary electron multiplier tube 45. A multiplying voltage for optimumobservation should be applied to the secondary electron multiplier tube45.

[0147]FIG. 16 shows the relationship between the correction voltage 124obtained by this operation and the output of the secondary electronmultiplier tube 45. This graph indicates that the greatest output can beobtained when the correction voltage 124 is equal to the charge-upvoltage of the specimen 12 (pointed to by arrow A). Substantially, dataof this graph is stored in memory of a control computer and used to setthe optimum correction voltage (pointed to by arrow A). This setting isrequired just after specimens 12 are changed and not required as far asan identical wafer is observed. However, this setting is required whenacceleration voltages or magnitudes of the primary electron beam arechanged.

[0148] Although the above embodiment applies a multiplying voltage foroptimum observation to the secondary electron multiplier tube 45, thisvoltage is controlled manually or automatically by an automatic circuitto get a sensitivity at which images of optimum contrast and brightnesscan be obtained in actual observation.

[0149] It is recommended to provide a level gauge and the like whichindicates the output of the secondary electron multiplier tube 45 tocontrol the surface voltage correcting voltage source. With the help ofthis level gauge, the operator can select an optimum correction voltageby adjusting the voltage source while monitoring the level gauge.

[0150]FIG. 17 shows a block diagram of a circuit which automaticallyadjusts a multiplication voltage given to the secondary electronmultiplier tube.

[0151] This circuit changes the DC voltage and adjusts themultiplication voltage to get a preset average output 125 (brightness)and a preset image amplitude (contrast of the scanning image). Theoutput of the secondary electron multiplier tube 45 is amplified by thepre-amplifier 126 and fed to the main amplifier 127. The amplitude ofthe output 125 of the main amplifier 127 is detected by the amplitudedetecting circuit 128 and controls the amplification voltage 129 to geta preset amplitude. The mean signal value is detected by the mean valuedetecting circuit 130 and controlled by the DC adjusting circuit 131 ofthe main amplifier 127.

[0152] It is needless to say that there are some brightness andamplitude values to be selected. The above circuit outputs a maximummultiplier voltage (limit of the power designing) when no secondaryelectron is entered. This circuit outputs less multiplier voltage as thesecondary electron input increases.

[0153]FIG. 18 shows the relationship between the surface-voltagecorrecting voltage 124 and the amplification voltage 129 when thecircuit of FIG. 17 is used for the second detector 35 of FIG. 14. Themultiplier voltage 129 becomes minimum at a correction voltage 124pointed to by an arrow. The secondary electron input becomes maximum atthis correction voltage 124. This method has a merit of performingdetermination of an adjusting voltage and adjustment of brightness andcontrast simultaneously.

[0154] As already explained, determination of an adjustment voltage isrequired not only for high-efficiency detection of secondary electronsbut also for determination of energy (acceleration voltage) applied tothe specimen, and further for determination of a magnification.

[0155] Further scanning electron microscopes have been used formeasurement of dimensions of processed parts of semiconductors. Thismeasurement requires a high precision of 1% or less. For example in theaforesaid example, let's assume that the specimen surface is charged at30 V when the primary electron beam of 2 kV undergoes deceleration bythe retarding voltage of 1.2 kV and hits a specimen with energy of 800V. In this case, the magnification change is 3.7%, which is a problem.However, the method in accordance with the present invention cansuppress the magnification change under 1% (0.6) as it can select anoptimum value at a precision of 5 V or less.

[0156] [Embodiment 12]

[0157] The example shown in FIG. 5 performs retarding by applying aretarding voltage to the specimen holder 100 and to the controlelectrode 27. This configuration forms a potential area having the samevoltage as the retarding voltage between the specimen holder 100 and thecontrol electrode 27. This seems that the retarding voltage is appliedto the specimen 12 in the potential area.

[0158] This method is effective for observation of specimens 12 whosesurface is coated with an insulating layer. This method applies aretarding voltage to such a specimen to cancel the charge. To observe aspecimen whose surface is not coated with an insulating layer, a meansis effective to apply a negative voltage to the specimen rest of thespecimen holder 100. Below are explained some other means to form adeceleration field.

[0159]FIG. 19 shows one of such means. This example employs an objectlens consisting of an upper magnetic pole 132 and a lower magnetic pole133 and applying an identical negative voltage (not visible in thedrawing) to both the lower magnetic pole and the specimen 12 to form adeceleration field. This lower magnetic pole is functionally the same asthe control electrode 27 in FIG. 5.

[0160]FIG. 20 shows another example of means to form a decelerationfield. This example forms a deceleration field between the specimen 12and an acceleration cylinder 9 to which a positive voltage is applied.This acceleration field works to accelerate secondary signals. Thesecondary signals accelerated by this deceleration field are deceleratedby a grounding potential area 135 and lose almost all of their energy inthis area 135. These secondary electrons without energy are deflectedout of the motion (beam axis) of the primary electron beam by adeflection field forming means (not visible in the drawing) anddeflected selectively.

[0161] The above embodiments of the present invention are all explainedusing scanning electron microscopes, but the present invention can beapplied to all devices for observing specimens by means of electronbeams, more particularly to measuring SEMs which measure widths ofpatterns formed on semiconductor devices and SEMs which repeatedlycompare patterns and check for defective patterns.

What we claim is:
 1. A scanning electron microscope comprising anelectron source, a lens for condensing the primary electron beam whichis emitted from said electrode, a detector for detecting electrons whichare generated by radiation of the primary electron beam condensed bysaid lens onto a specimen, a first decelerating means for deceleratingthe primary electron beam to be hit against said specimen, a seconddecelerating means for decelerating the electrons which are generatedfrom said specimen, and a deflector for deflecting the electrons whichare decelerated by said second deccelerating means towards saiddetector.